The present invention relates to a process for the detoxification of effluents containing cyanide and/or other oxidizable substances, utilizing hydrogen peroxide and sulfuric acid.
Purification of waste water and recovery of components thereof is of major importance in connection with protection of the environment. Effluents from a variety of industrial processes often contain cyanide in various forms such as simple or complex cyanide together with toxic metals. The recovery of such substances and the removal thereof from effluent is of paramount importance before such waste waters are discharged in order to avoid serious damage to the environment and adverse impact on public health and safety.
A number of methods have been developed over the years for the treatment of such effluents. One of the best among the known methods for treating effluents containing cyanide and heavy metals such as copper, zinc and nickel involves the oxidation of the cyanide contained in the waste water with hydrogen peroxide. Heavy metals contained in such waste effluents are usually removed by precipitation as a part of the overall process. These methods have been used successfully on a commercial scale for a number of years and are known to have a number of advantages over the even older methods such as treatment with chlorine because the excess hydrogen peroxide that is utilized decomposes to give only water and oxygen. In the older methods involving chlorine, undesirable salts are formed and introduced into the waste water. The utilization of hydrogen peroxide overcomes the disadvantages associated with methods which involve salt formation and the introduction of those salts into waste water.
In the case of effluents arising from ore-processing plants, the waste effluent often takes the form of so-called tailings pulp, or slurry, wherein the proportion of solids may be 50% or even higher. Hydrogen peroxide has been successfully used to treat such pulps and has been demonstrated on a commercial scale. In such operations, it has been found that the processing of some types of ore leads to tailings pulps which require relatively large amounts of hydrogen peroxide for treatment. In such cases, the economic viability of the detoxification process depends on reducing the consumption of effluent treatment chemicals as far as possible.
One method for reducing the consumption of hydrogen peroxide by accurate measurement of the oxidizing agent demand and control of the dosage thereof is shown in U.S. Pat. No. 4,416,786, assigned to the same assignee as the present application and which is relied on and incorporated herein by reference. This method has been shown to be successful in preventing unnecessary excess dosage of the hydrogen peroxide. It has also been found to be suitable for adapting the dosage to changes in concentrations of cyanide and other oxidizable substances in the effluent.
A further method for reducing the consumption of hydrogen peroxide, which is the subject of a patent application by the same assignee as the present application and which is relied on and incorporated herein by reference, involves the removal of magnetic material before treatment of the pulp, since magnetic components have been found to cause catalytic decomposition of the H.sub.2 O.sub.2 and a corresponding increase in consumption.
However, in some cases the consumption of hydrogen peroxide is still high, and further reduction in the consumption of hydrogen peroxide and consequently a reduction in the cost of waste water treatment is a worthwhile objective.
One procedure has been described in U.S. Pat. No. 3,900,555, which relies on the preparation of monoperoxysulfuric acid (also known as Caro's acid) using an apparatus described in U.S. Pat. No. 3,939,072, and subsequently adding the monoperoxysulfuric acid to the waste water, while simultaneously adding an alkali to the waste water in an amount suitable for neutralizing the acid. Although monoperoxysulfuric acid is an excellent oxidizing agent for the treatment of polluted water, the process described in U.S. Pat. No. 3,900,555, using the apparatus described in U.S. Pat. No. 3,939,072, suffers from the considerable disadvantage that the vessel in which the monoperoxysulfuric acid is prepared has to be cooled by passing a coolant through a water jacket in order to prevent overheating of the reactants and premature decomposition of the monoperoxysulfuric acid. A further disadvantage of the known process is that the formation of monoperoxysulfuric acid is a slow process, taking from 20 minutes to 3 hours to reach the equilibrium concentration of monoperoxysulfuric acid. The reaction mixture must be cooled the whole time to achieve good conversion of the reagents into monoperoxysulfuric acid.
The known reaction between sulfuric acid and hydrogen peroxide to form monoperoxysulfuric acid is generally depicted as follows: EQU H.sub.2 SO.sub.4 +H.sub.2 O.sub.2 .revreaction.H.sub.2 SO.sub.5 +H.sub.2 O
Theoretically, one mole of sulfuric acid reacts with one mole of hydrogen peroxide to yield one mole of monoperoxysulfuric acid. In practice, excess sulfuric acid is added, in order to maximize the amount of monoperoxysulfuric acid present in the equilibrium mixture.
Thus, for example, U.S. Pat. No. 3,900,555 describes the use of 150 ml/hour of sulfuric acid (66.degree. Be, or 96% H.sub.2 SO.sub.4 by weight) and 54 ml/hour of hydrogen peroxide (70% H.sub.2 O.sub.2 by weight) to produce 1 mole/hour of monoperoxysulfuric acid in the form of a 31% solution by weight. The mole ratio used in this example can be easily calculated, using the specific gravities of 96% H.sub.2 SO.sub.4 (1.8355) and 70% H.sub.2 O.sub.2 (1.288) and the molecular weights of H.sub.2 SO.sub.4 (98) and H.sub.2 O.sub.2 (34): ##EQU1##
U.S. Pat. No. 3,900,555 describes further the consumption of 608 g of 92% sulfuric acid and 164 g of 70% hydrogen peroxide, which is equivalent to a molar ratio for H.sub.2 SO.sub.4 :H.sub.2 O.sub.2 of 1.69:1.
Further information on the optimum mole ratio of H.sub.2 SO.sub.4 to H.sub.2 O.sub.2 for the conversion into monoperoxysulfuric acid is contained in British Patent No. 797,701, which teaches that the best mole ratio is 1.4:1, giving 94% conversion, and that if this ratio is reduced to 1.2:1 the yield is reduced to 89%, and that further reduction of the mole ratio to 1:1 results in conversion of only 63 or 75%, depending on the concentration of the reacting solutions.
The large excess of sulfuric acid employed in this process results in high cost, not only for the sulfuric acid itself, but also for the alkali needed to neutralize it in the detoxification process.